The present invention relates generally to scanning electron microscopes (SEMs), and more particularly to a fast scanning electron microscope (FSEM) capable of greater than 1 KHz framing rate with a large depth of focus and submicron resolution.
Light microscopes offering less than 2 micron spatial resolution and 25 nanosecond temporal resolution in strobed mode (2 kHz maximum real-time framing rate) have recently been developed (See, e.g., A. Ogura, N. Aizaki, and H. Terao, in "High-Speed Video Observation of Laser Recrystallization for Semiconductor-on-Insulator Fabrication," J. Appl. Phys. 65, 752 (1988).). However, transmission (TEM), reflection (REM) and scanning electron microscopes (SEM) offer spatial resolution superior to that of light microscopes. Transmission and reflection electron microscopes also provide temporal resolution of about 20 nanoseconds/frame, rivaling the temporal resolution of light microscopes. SEM, however, is the only type of electron microscope which offers both high spatial resolution and large depth of focus surface imaging, even for rough surfaces, of particular value for surface studies of material responses to dynamic loads at high magnifications, for example.
Recent advances in high-speed electron microscopy have used single-shot, microchannel-plate enhancement of light images obtained with transmission or reflection electron microscopes (See, e.g., O. Bostanjoglo, "Electron Microscopy of Fast Processes," Adv. Electron. Physics 76, 209 (1989).). Time resolution of about 20 nanoseconds is typical in these experiments. TEM, however, cannot image surfaces (except for monolayers), and requires thin specimens transparent, more or less, to electrons at the TEM operating voltage. Reflection electron microscopy offers surface imaging of sufficiently smooth surfaces but often presents greatly foreshortened images and depth of focus inferior to that of SEM.
As for temporal resolution in dynamic microscopy, Bostanjoglo [1989] has used pulsed-laser cathodes in TEM and the reflection electron microscope to attain temporal resolution of 20 nanoseconds. However, these experiments require the use of micro-channel plate intensified light images and are generally restricted to single-frame images inadequate to assess dynamic response. In addition, specimen beam damage is a consideration at the illumination levels required to produce usable images in such short illumination times. SEM framing rates are typically 1 Hz or less for atomic-scale scans due, primarily, to limited electro-mechanical scan bandwidth and extremely low imaging currents (higher currents would destroy the specimen). Rates of greater than 100 Hz are possible for SEM at lower magnifications. J. J. Wendolosky, K. H. Gardner, J. Hirschinger, H. Miura, and A. D. English, in "Molecular Dynamics in Ordered Structures: Computer Simulation and Experimental Results for Nylon 66 Crystals," Science 247, 431 (1990), quote NMR spectroscopy temporal resolution of 10 picoseconds. A. Ogura, N. Aizaki, and H. Terao, in "High-Speed Video Observation of Laser Recrystallization for Semiconductor-on-Insulator Fabrication," J. Appl. Phys. 65, 752 (1988), use a high-speed Kodak video recording system (1 kHz framing rate for full-frame, higher for partial frames) and laser illumination to obtain fast conventional light dynamic microscopy. A variety of real-time experiments with TEM and SEM has been performed at TV framing rates (30-50 Hz). Professor Kulabek (L. J. Balk, University of Duisberg, West Germany, written communication, Jan. 23, 1990.) has operated a secondary electron detector in a real-time SEM at a 16 MHz signal acquisition rate (about 5 times TV-rate). Also, strobed operation of SEM and the Auger microprobe permit less than 1 nanosecond temporal resolution, but only for cyclic phenomena. In strobed operation, Ogura cites temporal resolution of 25 nanoseconds using a pulsed laser illumination source.
Strobed SEM operation is well-suited to measuring integrated circuit potentials (using voltage contrast" techniques) when the circuit is operated cyclically, which is common for digital circuitry. It may also be possible to obtain very high-quality, short temporal resolution, strobed SEM images for material science studies by cyclic mechanical loading using, for example, sonic or ultrasonic propagation of either volume or surface waves in specimens of interest.
There is one other microscope (See, e.g., G. S. Kino and T. R. Corle, "Confocal Scanning Optical Microscopy," Physics Today, September, 1989, p. 55.) that is of interest in dynamic microscopy. This microscope is a "video-rate" confocal scanning light microscope. It has the potential for framing rates comparable to the lower end of our FSEM framing rates, with the spatial resolution of about 5000 Angstroms typical of conventional light microscopes but with superior sharpness due to improved edge resolution. In this device, fast laser beam scanning is obtained with acoustic-optic deflection capable of 200 MHz bandwidth (See, e.g., A. Yariv and P. Yeh, Optical Waves in Crystal, p. 383, Wiley and Sons, 1985.). The imaging signal is obtained with a scanned image dissector tube capable of 20 MHz signal acquisition rate and 0.5 MHz deflection bandwidth (See, e.g., S. Goldstein, "A No-Moving Parts Video Rate Laser Beam Scanning Type 2 Confocal Reflected/Transmission Microscope," J. Micros. 153, RP1 (1989).).
However, at higher loading rates (often single-pulse), hysteretic effects or catastrophic events such as brittle fracture or other sorts of material failure will require the use of a real-time, non-strobed, dynamic scanning electron microscope such as our FSEM. Because the scan beam in the SEM is an electron beam, maximum deflection rates typical of electrostatically-deflected oscilloscopes, permitting signal acquisition rates of approximately 10 GHz, allow for the potential of FSEM framing rates approaching 1 MHz.
As shown in FIG. 1, fast scanning for time-resolved electron microscopy (EM) in the prior art has only been conducted in a pulse-mode (non-continuous) on conventional (non-scanning) electron microscopes, and only stroboscopic imaging (periodic imaging) has been conducted on scanning electron microscopes (SEM).